Various alignment systems are known and can be found, inter alia, in the following U.S. Pat. Nos.: 4,540,277 Mayer et al.; 4,629,313 Tanimoto; 4,701,053 Ikenaga;
The prior art, however, fails to show a transmission alignment system usable for the direct alignment of transparent substrates. Mayer uses an adjustment plate which is inserted in the stage in the place of a wafer, and uses a separate alignment light. Tanimoto uses a stage detector that is positioned to one side of the wafer, and an alignment microscope. Ikenaga uses a detector which is below, not in, the stage, resulting in structural problems. The prior art does not disclose a detection system which uses transparent or transmissive substrates, in-stage detectors positioned so as to be underneath the substrate, and means for the reduction of reflection errors, nor does the prior art use a through the lens system (TTL), with in-stage sensors, for alignment.
For prior art detectors below the stage, such as Ikenaga, to be usable, a big cut-out area in the stage was required. This left inadequate gravity support for the wafer unless a glass support were used; and such support meant that the photocell would have to be too far removed from the wafer to be accurate. As a result, the use of reflective systems has become prevalent.
Our system permits the use of a more accurate transmission alignment system.